As background, discrete event simulation (DES) tools are designed to represent the operation of a system having a sequence of chronological events. Each of the events may occur at a particular instant in time and may result in a change of state in the system. Traditional DES tools have often used one of the following models: an event-driven model, a process-driven model, or an activity scanning model. Each of these models has advantages and disadvantages. For example, an event-driven model permits greater control and flexibility but is more complex to implement. As another example, a process-driven model may be easier for non-technical people to understand but may not be able to effectively model complicated systems.